Nice argument, but lets pick it apart for a minute.
I wholeheartedly agree with this. Although i haven't seen a crossbow used in an ingame melee situation, they sound about as powerful as they should be, minus the breaking potential. A crossbow armed dwarf fighting a weapon armed goblin of equal skill/toughness should stand no chance except maybe an occasional lucky blow, and for each lucky blow landed, it should have a pretty high chance of ruining the crossbow.
Crossbows used in melee are simply less damaging maces. If you look up the raws they have BASH 70 for damage. (swords have SLASH 120).
And thats a pretty broad statement to make. There are tons of other factors not mentioned such as what armour both combatants are wearing. If the dwarf was wearing leather and the goblin was wearing plate armour (as they commonly do) Equal skill and toughness would dictate that the goblin moves slower than the dwarf. The dwarf could evade then trip the goblin to the ground, this wouldn't even require a weapon, just some wrestling skill. Once the goblin is downed the dwarf could stab him with the bayonet conveniently stuck on the end of his crossbow.
No matter what menacing shaped piece of metal you attach to a crossbow it will still be about a tenth as effective as a proper melee weapon. It will also make the crossbow more unwieldy to carry and use. Think about it - dwarves are already fairly short, and attaching another foot of steel to the end of it would mean they wouldnt even be able to run with it pointing towards the ground in fear of it sticking in accidentally. And when they finally get to a spear wielding goblin, their l33t War crossbow will still be far shorter and much harder to use effectively due to its awkward spread of weight and lack of good handle.
Wrong. Just wrong.
1. A crossbow with a bayonet attached is infinitely better than just a crossbow and no bayonet. Since these are dwarves we are talking about, a "bayonet" would be a shortsword to them. A shortsword is a stabbing weapon and mounting it on the end of a crossbow simply improves your reach with it.
2. A crossbow with a bayonet welded to it is a spear that happens to fire bolts and not a crossbow. Proper bayonets are detachable and only fixed to the gun when needed. Most bayonets also double as fighting knifes, the rest are usually spikes of some sort.
3. So the shorty dwarves stumble over their spears where the goblin and human spearmen don't? BS. Goblins are even smaller than dwarves. They have the same size modifier AND thier equipment is considered "narrow" and will not fit even dwarves. Being able to run and fight with a long pointy object is a matter of discipline and skill. If the bayonet severely unbalanced the crossbow then maybe, but that would only be a problem if you made them out of lead. (or mounted a hammer, hehehe)
4. A firearm or crossbow built with bayonets in mind will be balanced to account for them. If your crossbows aren't working, you need a better bowyer.
5. When talking about firearms or crossbows the part we hold in our hands is called the GRIP. A handle is something you would find on a suitcase (thought some guns do have them). They come in two flavours, pistol and rifle. Its hard to say which the dwarves would use, and some crossbows in the middle ages didn't even have grips. They simply had a straight stock with a bent lever sticking out the bottom as a trigger.
6. Finally, what about the terrain here? Dwarf fortresses are made of tunnels and lots of them. ANY object that is very long will be extremely difficult to maneuver and fight with indoors. Currently this is not taken to account. A marksdwarf meeting a spearman in a tight tunnel could simply drop the bow and use the bayonet on its own. The spearman does not have this luxury.